funkydiplomat
Well-known member
A portion of a thread that started in the lab but i think belongs here.
the complete thread is here:
http://www.groupdiy.com/index.php?topic=10180
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NewYorkDave:
I just ordered four samples from the XSM series. I'll let ya know what I think when I get them.
I want to try the XSM10K/600 in a transformer-coupled version of my "one-bottle" preamp, replacing the plate resistor and coupling cap.
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funkydiplomat:
NYDAVE!!
here's a quick one for you... this is probably more drawing board material. If the thread gets out of hand, I'll start a new one over there....
When you replace the plate resistor and coupling cap in your design and send the B+ through the primary of the output tranny, does the DC resistance of the transformer become the plate resistor more or less? I've never done an output like this... only with a buffer stage AC coupled to a transformer. Mostly because it's a little bit cheaper. big-ass output iron that specs DC through the primary always seemed on the expensive side to me. Until edcor offered this... If it works out, I may try an amp like this.
joe
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NewYorkDave:
As a first approximation, ignoring hopefully "minor" parasitics: at AC, the plate sees the primary inductance and the reflected (stepped-up) impedance from the secondary in parallel--and this is in series with the copper resistance. At DC, it's just the copper resistance. Since the small copper resistance drops a lot less voltage than a plate resistor, the transformer is not a drop-in replacement. You have to redesign the output stage for the new DC conditions.
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funkydiplomat:
i see. can you make up the difference with a resistor? is it common for the resistor to be on the b+ side or the plate side of the primary? or would you re-design completely? A lower plate resistance value would make a lower output-Z at the expense of voltage gain for the stage, correct? just trying to make sure I'm on the right track here. You were using voltage feedback in your 'one-bottle' design, i think. you would still tap into the same place (between the plate and the tranny, instead of the plate and the plate load resistor) for the feedback? Occasionally I see feedback coming from odd places on output transformers, it may be from a power output stages I'm thinking of, though.
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the complete thread is here:
http://www.groupdiy.com/index.php?topic=10180
-------
NewYorkDave:
I just ordered four samples from the XSM series. I'll let ya know what I think when I get them.
I want to try the XSM10K/600 in a transformer-coupled version of my "one-bottle" preamp, replacing the plate resistor and coupling cap.
------
funkydiplomat:
NYDAVE!!
here's a quick one for you... this is probably more drawing board material. If the thread gets out of hand, I'll start a new one over there....
When you replace the plate resistor and coupling cap in your design and send the B+ through the primary of the output tranny, does the DC resistance of the transformer become the plate resistor more or less? I've never done an output like this... only with a buffer stage AC coupled to a transformer. Mostly because it's a little bit cheaper. big-ass output iron that specs DC through the primary always seemed on the expensive side to me. Until edcor offered this... If it works out, I may try an amp like this.
joe
-------
NewYorkDave:
As a first approximation, ignoring hopefully "minor" parasitics: at AC, the plate sees the primary inductance and the reflected (stepped-up) impedance from the secondary in parallel--and this is in series with the copper resistance. At DC, it's just the copper resistance. Since the small copper resistance drops a lot less voltage than a plate resistor, the transformer is not a drop-in replacement. You have to redesign the output stage for the new DC conditions.
--------
funkydiplomat:
i see. can you make up the difference with a resistor? is it common for the resistor to be on the b+ side or the plate side of the primary? or would you re-design completely? A lower plate resistance value would make a lower output-Z at the expense of voltage gain for the stage, correct? just trying to make sure I'm on the right track here. You were using voltage feedback in your 'one-bottle' design, i think. you would still tap into the same place (between the plate and the tranny, instead of the plate and the plate load resistor) for the feedback? Occasionally I see feedback coming from odd places on output transformers, it may be from a power output stages I'm thinking of, though.
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